A permanent magnet generator/motor generally includes a rotor assembly having a plurality of equally spaced magnet poles of alternating polarity around the outer periphery of the rotor or, in more recent times, a solid structure of samarium cobalt or neodymium-iron-boron. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternately, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor.
As high energy product permanent magnets having significant energy increases have become available at reduced prices, the utilization of the permanent magnet generator/motors has increased. Samarium cobalt permanent magnets having an energy product of twenty-seven (27) megagauss-oersted (mgo) are now readily available and neodymium-iron-boron magnets with an energy product of thirty-five (35) megagauss-oersted are also available. Even further increases of mgo to over 45 megagauss-oersted promise to be available soon. The use of such high energy product permanent magnets permits increasingly smaller machines capable of supplying increasingly higher power outputs.
One of the applications of a permanent magnet generator/motor is referred to as a turbogenerator which includes a power head mounted on the same shaft as the permanent magnet generator/motor, and also includes a combustor and recuperator. The turbogenerator power head would normally include a compressor, a gas turbine and a bearing rotor through which the permanent magnet generator/motor tie rod passes. The compressor is driven by the gas turbine which receives heated exhaust gases from the combustor supplied with preheated air from recuperator.
In order to start the turbogenerator, electric current is supplied to the stator coils of the permanent magnet generator/motor to operate the permanent magnet generator/motor as a motor and thus to accelerate the gas turbine of the turbogenerator. During this acceleration, spark and fuel are introduced in the correct sequence to the combustor and self-sustaining gas turbine conditions are reached.
At this point, the inverter is disconnected from the permanent magnet generator/motor, reconfigured to a controlled 60 hertz mode, and then either supplies regulated 60 hertz three phase voltage to a stand alone load or phase locks to the utility, or to other like controllers, to operate as a supplement to the utility. In this mode of operation, the power for the inverter is derived from the permanent magnet generator/motor via high frequency rectifier bridges. A microprocessor can monitor turbine conditions and control fuel flow to the gas turbine combustor.
In order to produce three phase, four wire utility grade power from the turbogenerator, there must be allowance for single phase loads. In order to accomplish this, the turbogenerator power converter must include an inverter which has a neutral. One way that pulse width modulated inverters establish a neutral is by providing a fourth channel of Integrated Gate Bipolar Transistors (IGBT) switches and associated gate drives and output filter inductor. By switching the IGBT's in this fourth channel to a fifty percent (50%) duty cycle, an artificial neutral connection is created which is exactly at the mid point of the DC bus potential.
An example of such a system is described in U.S. patent application Ser. No. 924,966, filed Sep. 8, 1997 (issued May 5, 1999 as U.S. Pat. No. 5,903,116) by Everett R. Geis and Brian W. Peticolas entitled "Turbogenerator/Motor Controller", assigned to the same assignee as this application and incorporated herein by reference. While this approach to establish a neutral is quite functional, its cost in increased components, such as the solid state transistors, associated gate drives, and output filter inductor, makes such a system relatively expensive to produce.